Perfluoroalkyl iodides having about 6 to 12 carbon atoms are useful as ingredients for surfactants and for water- and oil-repellents for fibers.
For the production of perfluoroalkyl iodides, industrially employed are processes using telomerization as shown in the following reaction formula:RfI+nCF2═CF2→Rf(CF2CF2)nIwherein Rf is a perfluoroalkyl having 1 to 6 carbon atoms and n is an integer from 1 to 4.
This reaction is known to proceed with heating. German Patent Publication No. 1,443,517, for example, discloses a method wherein a reaction is conducted at a temperature of 250 to 800° C. under a pressure of 2 mmHg to 5 atmospheres with a residence time of 1 hour or less. Such a thermal reaction, however, poses a problem with the production of large amounts of perfluoroalkanes, which are the dimerized products of the perfluoroalkyl radicals generated in the reaction.
Japanese Unexamined Patent Publication No. 305995/1994 discloses a method of thermal telomerization at a high temperature of about 300 to 360° C. In this method, generated are by-products such as iodine and perfluoroalkanes produced by the reactions among the telomers. Especially due to the generation of iodine, reactor corrosion, clogging of pipes and similar components, and other problems are likely to occur. Furthermore, the introduction of tetrafluoroethylene as a taxogen at high temperatures poses a safety problem.
Meanwhile, a variety of catalysts have been developed to conduct telomerization at lower temperatures.
For example, UK Patent No. 1,535,408, U.S. Pat. No. 5,068,471, etc., disclose processes of telomerization through the use of free-radical generators. In these processes, however, perfluoroalkyl radicals react with the free-radical generators and produce, as by-products, hydrogen-containing organic compounds represented by RfH, wherein Rf is a C1-6 perfluoroalkyl.
To avoid the generation of undesired long-chain telomers (compounds represented by the formula Rf(CF2CF2)nI, where n is 5 or more), the concentration of starting telogens (RfI) is generally increased and the concentration of taxogens is decreased. As a result, the conversion to the desired medium-chain telomers (compounds represented by the formula Rf(CF2CF2)nI, where n is 1 to 4) is low, and the starting telogens (RfI) are recycled by distillation.
However, it is difficult to separate the by-product (i.e., RfH) from the starting compound (i.e., RfI). Thus, when telomerization is continuously carried out, RfH causes a disadvantage of decreasing reaction efficiency due to its accumulation in the telogen.
Chen, et al., (Preliminary Note, Journal of Fluorine Chemistry 36 (1987), pp. 483-489) disclose the use of copper powder as a catalyst for telomerization. This reaction proceeds at a low temperature of 80 to 100° C., and moreover, is advantageous for achieving a reaction time that is shorter than that of telomerization conducted at high temperatures.
Japanese Unexamined Patent Publication No. 239335/1996 describes zinc, magnesium, vanadium, rhenium, rhodium, ruthenium, platinum and silver as telomerization catalysts. Moreover, Japanese Unexamined Patent Publication No. 239336/1996 and other publications disclose, in copper-catalyzed telomerization, a method employing other transition metal as a co-catalyst.
However, even when the aforementioned various catalysts are used, their catalytic activities are still insufficient, and the selectivity for medium-chain telomeric compounds, in which n is 4 or smaller, is also unsatisfactory.